I'm yarding out the 20 year old charger inverter (inverter giving out bad juice) and replacing it with a new Pro Mariner 50 Amp charger. The installation instructions call for 6 AWG wire for a run under 15' ( my run is about 8'). The old ginormous charger inverter had 2 AWG wire and ground to the battery with a 250A fuse! The inline fuse for the new charger is 60A.

I think what we have here is new technology. I don't want to make new cables if I can leave the 2 AWG in there. Will that hurt anything? The charger is going to be for a bank of 4 CG batteries (flooded cell).

Anything to do with battery DC cables to switches and busses, the PO's used 2 AWG for everything.

Go to the Blue Sea site and use their circuit tables to calculate the size of the breaker/fuse. Remember, you are protecting the cable, not the device. The device will have its own fuse. The usual reason for not using bigger wire than necessary is cost. Your wire cost is sunk so no point in changing it.

Figure out the correct fuse size.

__________________
Don't believe everything that you think.
What are we offended about today?

You don't have to use wiring tables to figure out the fuse size. The minimum size fuse which will work fine is based on the maximum current that the wire has to pass- so a 60 amp fuse gives you 20% margin.

It won't hurt to use a bigger fuse up to the ampacity of the wire which is 330A for 2/0, but it is always best to use the minimum.

Oh, and put the fuse near the battery. That is where hundreds of amps of current will originate from if there is a fault. The 50A charger is limited to that current and your wire will handily carry that much forever.